This invention relates generally to a method for determining ionic species, particularly anionic species in aqueous solution, particularly pulp process liquors of cellulosic pulp manufacturing processes, by near infrared spectrophotometry and more particularly to the use of an on-line method for determining concentration parameters of said process liquors, and subsequent control of said cellulosic pulp manufacturing process by use of said determined parameters.
Kraft pulping is performed by cooking wood chips in a highly alkaline liquor which selectively dissolves lignin and releases the cellulosic fibers from their wooden matrix. The two major active chemicals in the liquor are sodium hydroxide and sodium sulfide. Sodium sulfide, which is a strong alkali, readily hydrolyses in water to produce one mole of sodium hydroxide for each mole of sodium sulfide. The term xe2x80x9csulfidityxe2x80x9d is the amount of sodium sulfide in solution, divided by the total amount of sodium sulfide and sodium hydroxide and is usually expressed as a percentage (% S) which varies between 20 and 30 percent in typical pulping liquors. The total amount of sodium hydroxide in solution, which includes the sodium hydroxide produced as the hydrolysis product of sodium sulfide, is called either xe2x80x9ceffective alkalixe2x80x9d (EA), expressed as sodium oxide, Na2O before pulping, or residual effective alkali (REA) after pulping. Timely knowledge of these parameters would enable good control of the pulping process.
At the beginning of the kraft process, xe2x80x9cwhite liquorxe2x80x9d is fed to a digester. This white liquor contains a high amount of effective alkali up to 90 g/L, as Na2O. At intermediate points in the digester, spent liquor, or xe2x80x9cblack liquor,xe2x80x9d is extracted from the digester. This spent liquor contains low levels of effective alkalixe2x80x94less than 30 g/L, as Na2O and also contains large amounts of organic compounds which, generally, are burned in a recovery furnace. Resultant inorganic residue, called smelt, is then dissolved to form xe2x80x9cgreen liquorxe2x80x9d which has a low concentration of effective alkali and a high concentration of sodium carbonatexe2x80x94up to 80 g/L, as Na2O. White liquor is regenerated from the green liquor by causticizing the carbonate through the addition of lime. After the recausticizing operation, a small residual amount of sodium carbonate is left in the white liquor. The combined amount of sodium hydroxide, sodium sulfide and sodium carbonate is called total titratable alkali (TTA). The causticizing efficiency (CE) is usually defined as the difference in the amounts, as Na2O of sodium hydroxide between the white and green liquors, divided by the amount, as Na2O of sodium carbonate in the green liquor. Sodium sulfate, sodium carbonate and sodium chloride represent a dead load in the liquor recycling system. The reduction efficiency (RE) is defined as the amount, as Na2O of green-liquor sodium sulfide, divided by the combined amounts, as Na2O, of sodium sulfide, sodium sulfate, sodium thiosulfate and sodium sulfite in either green liquor or the smelt.
The timely knowledge of the white-liquor charge of EA and of black-liquor EA would close the control loop in the digester and optimise for example, production and product quality and chemical utilization, of alkali and lime consumption. The control of sodium sulfide, TTA and of non-process electrolytes, such as sodium chloride and potassium chloride would also have a beneficial impact on closed-cycle kraft-mill operations. For example, environmentally-driven reduction of sulfur losses generally increases liquor sulfidity, thereby creating a sodium:sulfur imbalance that needs to be made up through the addition of caustic soda. Another important need is the control of TTA in green liquor, which is most easily done by adding weak wash to a smelt dissolving tank. The value of the green-liquor TTA is important because it is desirable to maintain the TTA at an optimal and stable level so as to avoid excess scaling while obtaining a high and stable white liquor strength. The ongoing development of modern chemical pulping processes has thus underscored the need for better control over all aspects of kraft-mill operations and more efficient use of all the chemicals involved in the process by knowledge of the concentration of aforesaid species in the liquors.
Sodium carbonate is difficult to characterise and quantify in situ because of a current lack of on-line sensors which can tolerate long-term immersion in highly alkaline liquors. Important economic benefits could result from causticizing control with a reliable sensor for sodium carbonate. Accurate causticization is critical for the uniform production of high-strength white liquor in that adding too much lime to the green liquor produces a liquor with poorly settling lime mud, whereas adding too little produces a liquor of weak strength. Determining the relative quantities of EA and carbonate in green and white liquor is thus important for controlling the causticizing process.
The recovery furnace of a recovery process produces a molten salt (smelt) that contains, in part, oxidized and reduced sulfur compounds. This smelt is dissolved in water to produce raw green liquor. The oxidized sulfur compounds are mainly in the form of sodium thiosulphate (Na2S2O3) and sodium sulfate (Na2SO4), while the reduced sulfur is in the form of sodium sulphide (Na2S). Since only the sodium sulphide is useful in the pulping process, it is desirable to keep the proportion of sulfur that is reduced, known as the reduction efficiency, as high as possible. Timely measurement of sulphate and thiosulphate in the raw green liquor would allow improved control of the recovery boiler""s reduction efficiency.
Some mills produce fully oxidized white liquor for use in the bleach plant. In this process, the sodium sulphide ions in the white liquor are first partially oxidized to sodium thiosulphate (Na2S2O3), and then filly oxidized to sodium sulfate (Na2SO4). Timely measurement of the sodium thiosulphate concentration that is remaining in the liquor would allow improved control of the oxidation process.
It is known that an increase in carbohydrate yield in a kraft cook can be achieved by the addition of sodium polysulphide to conventional white liquor. Reference is made to this process in an article published in Svensk Papperstidn, 49(9):191, 1946 by E. Haegglund. Sodium polysulphide acts as a stabilizing agent of carbohydrates towards alkaline peeling reactions. Thus, polysulphide-cooking results in a significant pulp yield gain, which provides increased pulp production, and reduces the cost of wood chips.
A common method for producing polysulphide is to convert the sodium sulphide already present in the white liquor to polysulphide by an oxidation process. Several variants of this method are reported by Green, R. P. in Chemical Recovery in the Alkaline Pulping Process, Tappi Press, pp. 257 to 268, 1985 and by Smith, G. C. and Sanders, F. W. in the U.S. Pat. No. 4,024,229. These procedures generally involve redox and catalytic or electrochemical processes.
A typical polysulphide process is carried out in the recausticizing tank, which has a residence time of approximately 60 minutes. An example of such a process is described in G. Dorris U.S. Pat. No. 5,082,526. The main product, polysulphide, is produced through an oxidation reaction which also creates sodium thiosulphate through over-oxidation. Process conditions must therefore be controlled so that a maximal amount of polysulphide is produced. With a closed-loop control system, this is best achieved with a minimum sampling rate of 4 samples per unit of residence time. The traditional methods presently available for polysulphide are based on wet chemical methods and all take several hours. Therefore, they are not suitable for control methods. A spectrophotometric method had been reported by Danielsson et. al, Journal of Pulp and Paper Science, 22(6), 1996. Unfortunately, this method must either use a short pathlength, on the order of 50 xcexcm, or use diluted liquors, both of which are not practical for online applications. A method that does not require dilution is desirable.
Traditionally, hydrogen peroxide has been used in chemical pulp bleaching for providing marginal increases in brightness near the end of the bleaching process. More recently, the use of hydrogen peroxide as a bleaching agent for kraft pulp has been growing rapidly because of the elimination of elemental chlorine from the chlorination stage and the implementation of oxygen delignification. The use of peroxide reinforces the oxidative extraction stage by delignifying as well as bleaching the pulp in the EOP stage, and enables the preceding chlorine dioxide stage (D) to be run at a much lower chlorine dioxide charge, thereby preventing the formation of environmentally harmful by-products such as dioxins. This practice also allows the mill to maintain its final brightness target.
Peroxide bleaching is strongly affected by pH, which must be adjusted and buffered at around 10.5 for best results. The pH of the bleach liquor is usually controlled by the addition of sodium hydroxide. A chelating agent such as di-ethylene-tetra-amine-penta-acetic acid (DTPA) or sodium silicate is also added, which act both as a stabilizer and as a buffering agent in the peroxide bleaching system. DTPA scavenges trace transition metals, such as manganese, which decompose hydrogen peroxide. Magnesium sulfate is added as a final stabilizing agent during the pulp-bleaching step. Since hydrogen peroxide is an expensive chernical, its concentration in the bleach liquor (typically between three to five percent by volume) must be carefully controlled so as to yield maximal benefit from its use.
Chlorine dioxide solution (ClO2(aq)) is a bleaching agent commonly used in the production of chemical pulps. Chlorine dioxide is generated by reacting sodium chlorate (NaClO3) with a reducing agent, typically liquid methanol (CH3OH) or sulfur dioxide (SO2) gas. A strong acid, typically sulfuric acid (H2SO4) or hydrochloric acid (HCl), is normally present to increase the reaction rate.
Efficient production of chlorine dioxide requires that the chlorate and acid concentration in the generator be kept at optimum levels. If the either the chlorate or acid concentration varies, undesirable chemical reactions occur that reduce generator efficiency. Timely knowledge of the chlorate and acid concentrations in the generator and in the feed streams would allow improved control of the chlorine dioxide generator.
Current control technology for chlorine dioxide generation from chlorate and sulphuric acid consists of regularly monitoring the generated chlorine dioxide by UV spectrometry, and using the results for feedback control of the process. However, chlorate and sulphuric acid are only very sporadically measured in the laboratory by titration, thereby leading to untimely and incomplete feed-forward control of the generating process. Titration is currently the method of choice, since the generating liquor contains a high level of bubbles and solids such as sesquisulphate, and is generally thought not to be suitable for on-line spectrometric analysis.
The choice of infrared-transparent optical materials for use in this application is rather limited. Only diamond and fused silica can withstand strongly acidic liquors. In the mid-infrared, a short pathlength must be used because of strong absorption by the fundamental bands of water. Normally, ATR would be the technique of choice because of the short pathlength and the strong fundamental bands for chlorate. However, silica cannot be used since it is opaque below 2200 cmxe2x88x921. Also, diamond is susceptible to scaling, and strongly absorbs in the region used for monitoring sulfate and chlorate if more than two reflections are used, which makes it unsuitable for quantitative analysis due to the lack of precision with the absorbance measurements.
On the other hand, in the near-infrared region, one can use a transmission cell with a relatively long pathlength. This pathlength should be long enough to permit adequate determination of the analyte for process control purposes. The presence of bubbles and solids would discourage a person ordinarily skilled in the art of ClO2 generation from investigating the relatively long pathlength needed for a successful application.
Contrary to expectations, we have found that a near-infrared on-line spectrometric method is indeed possible for the analysis of chlorate and sulfuric acid. This method enables mill personnel to implement effective feed-forward control and to safely operate the generator under optimal conditions.
Various methods of on-line measurements of either EA or sodium hydroxide have been proposed. The use of conductivity methods for green and white liquors is well-established as a pulp and paper technology. Unfortunately, conductivity probes are prone to drift due to scaling, as well as interferences from other ionic species. Therefore, these devices require frequent maintenance and re-calibration. An early example of such measurements describes a method that can determine the EA by neutralizing hydroxide ions with carbon dioxide (1). The conductivity of the solution is measured before and after treatment. The difference in conductivities is proportional to the hydroxide ion concentration of the liquor. High levels of sodium hydroxide, however, will increase the neutralizing time. In white liquors, this time is too long for effective process control purposes. Chowdhry (2) describes an analysis of kraft liquors that uses differences in conductivity before and after precipitation of carbonates using BaCl2, an approach which is not practical.
However, even though conductivity probes may not be suitable for on-line measurements of EA in white or green liquors, this kind of sensor is also used with the liquor produced during the early stages of the pulping in upper-recirculation digester lines. An example of a successful commercial version of an automatic titrator (3) involves titrating alkali with sulfuric acid until no change in conductivity is observed. This determination is straightforward and works very well for the impregnation and early stages of the cook, but not for the extraction stage. With extraction liquors, a more complex pattern is observed when significant quantities of organic acids and black-liquor solids appear in the liquor, and the end-point determination becomes more difficult near the end of the cook. On-line titration methods used in pulp mills suffer from frequent maintenance problems. Thus, most mill-site measurements still rely on standard laboratory methods.
At present, control of digesters is performed by keeping the chip and white liquor feeds at preset levels. These levels are determined by the overall production rate, and control is achieved by adjusting the temperature profile of the cook and determining the resultant blow-line kappa number. The philosophy behind this strategy is that alkali consumption during the removal of lignin is proportional to chip feed at a given kappa number. Alkali not consumed in the impregnation phase is then available for the bulk removal of lignin that occurs in the pulping zone. This is usually performed by predicting the pulp yield with the H-factor (4). The disadvantage of this method is that it assumes uniform chip moisture content, pH and density, as well as digester temperature, etc. Since the pulp must be analysed in the laboratory for lignin content, this makes it difficult to close the control loop in a timely manner. Ideally, a much better way of controlling digester operations would be to measure the EA concentration in black liquor directly on-line at an appropriate time in the cooking process on both the upper and lower (main) recirculation loops in the digester, as well as the REA concentration on the extraction line at the end of the cook. An on-line method that would give a direct measurement of the EA throughout a cook is therefore needed.
Methods relying on spectroscopic methods have been proposed because of the limitations of titration and conductivity methods for liquor analysis. It is known that hydrosulfide ions absorb very strongly in the ultraviolet at 214 nm (5, 6, 7). However, this absorption is so strong that a very small pathlength, i.e. less than 10 microns is needed to get a measurable signal which yields a linear calibration curve (8). A cell with such a small optical path is prone to plugging and, hence, not practical for on-line applications. Extensive 1:1xc3x97103 or 104 dilution is practiced, which results in inaccurate results and increases the risk of sulfide being oxidized.
The dilution approach has also been used in techniques such as capillary zone electrophoresis which use UV detectors (9, 10). Errors in sulfidity measurements exceeding 50% were reported. Accordingly, a method which does not need dilution is needed.
Infrared spectroscopy can distinguish between the inorganic and organic components of liquors and a number of infrared methods have been proposed. Faix et al (11) propose a method for organic compounds in black liquor, based upon on-line infrared attenuated reflectance (ATR) measurements between 1400 and 1550 cmxe2x88x921. A similar method for kappa number determination (12) correlates the increase in the integrated band intensity at 1118 cmxe2x88x921 with decreasing kappa number. Neither of these methods can be used for process control because of interferences from carbohydrates and uncertainties in the value of process variables such as liquor-to-wood ratio. Leclerc et al. (13, 14, 15, 16) teach that one can measure EA and dead-load components in kraft liquors with FT-IR ATR, and that one can use these measurements to control the operations of important process units involved in the manufacture of kraft pulp such as the digester, recausticizers and recovery boiler. However, ATR optical reflecting elements immersed in very alkaline liquors, and/or acidic or oxidizing cleaning solutions, are prone to be vulnerable to etching and/or scaling of their surface, which necessitates frequent replacement, re-polishing and re-calibration of the elements. Materials that are resistant to caustic, acidic, or oxidizing environments are few and cannot be used for ATR measurements in the mid-infrared region of interest due to infrared absorption of the material itself. ATR elements have also slightly differing optical paths and surface properties that exhibit memory, which makes the transfer to other instruments of calibrations developed on one instrument very difficult to achieve without substantial expenditures of time and labour.
Recent advances in FT-IR instrumentation and software have made possible the more widespread use of the near-infrared region of the spectrum for deterinig aqueous components such as dissolved electrolytes. Each ionic species causes a unique and measurable modification to the water bands that is proportional to its concentration. Advantages over previous techniques include: no sample preparation, short measurement times, relatively long optical paths and the possibility of using fiber-optic technology for real-time, in situ measurements. Also, temperature effects and interferences by other cations and anions can be modeled in this spectral region through the use of partial least-squares (PLS) multi-component calibration techniques. PLS is a well-known multi-component calibration method (17, 18). This method enables one to build a spectral model which assumes that the absorbance produced by a species is linearly proportional to its concentration. This has been shown by (19, 20, 21, 22, 23). However, because of its relatively intense water bands, the spectral region situated from 4000 to 8000 cmxe2x88x921 is only suitable for optical paths ranging from 0.5 to 1.5 mm, a limitation which precludes the accurate determination of weakly absorbing electrolytes such as carbonate, sulfide and chloride. Sodium hydroxide, on the other hand, generates a strong signal that is easily detectable in this region (24, 25, 26). The concentration of dissolved electrolytes, such as sodium hydroxide, carbonate and chloride concentrations in aqueous streams, such as seawater or white liquor have been measured. Accurate results were obtained for hydroxide but not for the other ions. Similar results were obtained more recently (27) with a PLS calibration. The correlation data obtained for sulfide and carbonate are not reliable, and cannot be used as a basis towards developing a method for controlling the manufacture of cellulosic pulp. A near-infrared PLS method, which can measure sodium sulfide and TTA with an accuracy of 1 to 2 g/L has been described (28). The calibration method, however, could not distinguish between sodium carbonate and sodium hydroxide because of the similar spectral signatures produced by these two ions, as well as the relative weakness of the carbonate spectrum. Reference 24 through 28 demonstrate that hydroxide is easy to measure in the range 4000 to 8000 cmxe2x88x921, while other components such as carbonate and sulphide are not. The results obtained (27, 28) strongly suggest that a control method for a pulp manufacturing process based on the simultaneous and separate determination of hydroxide, carbonate and sulfide would be very difficult with the small-bore flow cell used for their work. This type of flow cell would also be susceptible to plugging by suspended solids and fibers, thereby rendering the method unworkable. The spectral region situated from 8000 to 12000 cmxe2x88x921 is more amenable to the use of longer optical paths ranging from 3 to 20 mm, which makes it much easier to couple a wide-bore flow cell to any system of pipes used in the mill. For example, (23, 29) a PLS calibration has been used to resolve the hydroxide and chloride ion spectrum near 10500 cmxe2x88x921. In both cases, however, the range of concentration was extremely wide (0 to 5 moles/L), the spectra were somewhat noisy, and the precision was no better than 5 g/L for both species. For the spectral information to be useful for process control engineers, the correlation data must be accurate to within one percent and the level of precision, in the range of 0.5 to 1 g/L. The level of precision reported is, thus, inadequate for process control.
A recent publication (30) broadly discloses a method of controlling the causticizing reaction for producing a white liquor having multiple white liquor components from a green liquor having multiple green liquor components, comprising the steps of measuring a characteristic of each of said green liquor components; measuring a characteristic of each of said white liquor components; evaluating said green liquor component characteristics and said white liquor component characteristics in a non-linear, application adaptable controller to produce a causticizing control signal; and controlling said causticizing reaction responsive to said causticization control signal to produce white liquor wherein the characteristics are generally measured by near infrared or polarographic measurement devices and evaluating the characteristics in a non-linear, application adaptable controller to produce a causticizing control signal for controlling the amount of time to a shaker. However, the specific multiple component liquid process analyzer of use in the disclosed process would require a pathlength of less than 3 mm at 1100 to 2200 nm to avoid complete saturation of the incident light beam by water molecules in the sample.
There is, therefore, a need for the rapid determination of effective alkali, residual alkali, sodium sulfide and sodium carbonate, particularly, in pulping process liquor by spectrophotometric means which provide for a process liquor pathlength of greater than 3 mm without saturation of the incident radiation beam by water molecules of the sample.
1. U.S. Pat. No. 3,553,075xe2x80x94Rivers
2. U.S. Pat. No. 3,607,083xe2x80x94Chowdhry
3. U.S. Pat. No. 3,886,034xe2x80x94Noreus
4. K. E. Vroom, Pulp Paper Mag.Can., 1957, 58(3), 228
5. U.S. Pat. No. 5,582,684xe2x80x94Holmquist and Jonsson
6. D. Peramunage, F. Forouzan, S. Litch. Anal. Chem., 1994, 66, 378-383
7. Paulonis et al. PCT Application WO 91/17305. Liquid Composition Analyser and Method
8. Paulonis et Krishnagopalan. Kraft White and Green Liquor Composition Analysis. Part I: Discrete Sample Analyser. J. Pulp Paper Sci., 1994, 20(9), J254-J258
9. Salomon, D. R., Romano, J. P. Applications of Capillary Ion Analysis in the Pulp and Paper Industry. J. Chromatogr., 1992, 602(1-2), 219-25
10. Rapid Ion Monitoring of Kraft Process Liquors by Capillary Electrophoresis. Process Control Qual., 1992, 3(1-4), 219-271.
11. U.S. Pat. No. 4,743,339. Faix et al.
12. Michell. Tappi J., 1990, 73(4), 235.
13. Leclerc et al. J. Pulp Paper Sci., 1995, 21(7), 231
14. U.S. Pat. No. 5,282,931xe2x80x94Leclerc et al.
15. U.S. Pat. No. 5,364,502xe2x80x94Leclerc et al.
16. U.S. Pat. No. 5,378,320xe2x80x94Leclerc et al.
17. Haaland, D. M. and Thomas, E. V. Anal. Chem., 60(10): 1193-1202 (1988)
18. Haaland, D. M. and Thomas, E. V. Anal. Chem., 60(10): 1202-1208 (1988)
19. Lin and Brown. Appl. Spectrosc. 1992, 46(12), 1809-15
20. Lin and Brown. Environ. Sci. Technol. 1993, 27(8), 1611-6
21. Lin and Brown. Anal. Chem., 1933, 65(3), 287-92
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23. Lin and Brown. Appl. Spectrosc. 1993, 47(2), 239-41
24. Watson and Baughman. Spectroscopy, 1987, 2(1), 44
25. Hirschfeld. Appl. Spectrosc., 1985, 39(4), 740-1
26. Grant et al. Analyst., 1989, 114(7), 819-22
27. Vanchinathan, S., Ph.D. Thesis. Modeling and control of kraft pulping based on cooking liquor analysis, Auburn University, 1995. Tappi J., 1996, 79(10):187-191
28. U.S. Pat. No. 5,616,214. Leclerc
29. Phelan et al. Anal. Chem., 1989, 61(3), 1419-24
30. WO98/10137xe2x80x94Fisher Rosemont Systems, Inc.; Mar. 12, 1998.
It is an object of the present invention to provide a rapid method for determining the concentration of OHxe2x88x92, CO3xe2x95x90 and HSxe2x88x92 species in aqueous solution, particularly in solutions containing all three species.
It is a further object to provide a rapid method for determining the concentration of organic species present in a pulping process liquor, particularly, in the presence of at least one of the species selected from OHxe2x88x92, CO3xe2x95x90 and HSxe2x88x92.
It is a further object to provide a rapid method for determining the concentration of effective alkali, residual alkali, sodium sulfide, sodium carbonate and dead-load components such as chloride and dissolved organic species in pulp liquors.
It is a further object of the present invention to provide a rapid method for determining the concentrations of sulphate and thiosulphate in the presence of OHxe2x88x92, CO32xe2x88x92, or HSxe2x88x92, particularly in solutions containing two or more of these species.
It is a further object of the present invention to provide a rapid method for determining the concentrations of polysulphide in the presence of OHxe2x88x92, CO32xe2x88x92, and HSxe2x88x92, particularly in solutions containing all four species.
It is a further object to provide a rapid method for determining the concentration of peroxide ions in the presence of OHxe2x88x92, CO32xe2x88x92, and HSxe2x88x92, particularly in the presence of two or more of these species.
It is a further object to provide an improved method for the analysis of chlorate and sulfuric acid.
It is a yet further object to provide said rapid process which does not need frequent equipment maintenance, sample pretreatment or chemical reagents.
It is a still yet further object to provide said method which, optionally, allows a plurality of pulp liquor process streams to be multiplexed to a single analyser in a fibre-optic network.
It is a further object to provide apparatus for effecting said methods.
Accordingly, the invention provides in one aspect a method for determining the concentration of hydrogen ion, organic anionic species and anionic species selected from the group consisting of OHxe2x88x92, CO3xe2x95x90, HSxe2x88x92, ClO3xe2x88x92, SO4xe2x95x90, S2O3xe2x95x90, polysulphide and peroxide in an aqueous sample solution, said method comprising subjecting said solution to near infrared radiation at a wavelength region of wave numbers selected from about 7,000 to 14,000 cmxe2x88x921 through a solution path length of at least 3 mm to obtain spectral data for said solution; obtaining comparative spectral data for said anionic species at known concentrations in aqueous solutions; and correlating by multivariate calibration the relationships between said spectral data of said sample solution and said comparative spectral data to determine said concentration of said anionic species in said sample solution.
Preferably, the wavelength is selected from 7,000 to 12,000 cmxe2x88x921, and more preferably, 9,000 to 12,000 cmxe2x88x921.
The spectral data is preferably obtained by transmittance spectrophotometry, and more preferably, from a transmission cell. The relationships between the spectral data of the sample and the comparative spectral data are, preferably, obtained with a partial-least-squares multivariate calibration.
In a preferred aspect the invention provides a process for controlling the operation of individual unit operations within a cellulosic pulp manufacturing process, which comprises the steps of:
subjecting samples of process liquors to near infrared radiation at a wavelength region of wavenumbers from about 7,000 to 14,000 cmxe2x88x921 to produce measurements of said liquor;
recording the spectrum of different mixture solutions of synthetic and process liquors having known concentration parameters;
correlating by multivariate calibration the relationships between the spectra of the process liquor samples and the different mixture solutions of known concentration parameters so as to simultaneously determine concentration parameters in the process liquor samples; and
adjusting the individual unit operations of the cellulosic pulp manufacturing process as required by controlling at least one process parameter to bring the final product of said unit operation to a desired value, wherein said final product is determined in part by concentration parameters in said process liquors, as determined by the near infrared measurements of said concentration parameters.
Thus, the invention, in a preferred aspect, provides a rapid method for the control of a cellulosic pulp manufacturing process via on-line measurement of chemical concentration parameters in process liquor streams with near infrared radiation. The method eliminates the need for (i) manual sampling, (ii) frequent equipment maintenance, (iii) a dedicated instrument at each sampling point, (iv) compensation for instrumental drift, and, optionally, (v) an environmentally controlled spectrometer housing near the sampling location(s). The method includes the steps of (i) withdrawing samples of a process liquor stream from a cellulosic pulp manufacturing process, (ii) subjecting the samples to near-infrared spectrophotometry over a predetermined range of wavenumbers so as to produce spectral measurements which determine the concentrations of different combinations of chemical components, (iii) correlating by multivariate calibration the relationships between the spectral measurements of unknown samples and the spectral variations shown by different combinations of chemical components of the process liquor so that concentration parameters can be accurately determined for typical levels of chemical components present in the process liquor, and (iv) controlling at least one process parameter so as to obtain optimal operation of the cellulosic pulp manufacturing process.
The method of the present invention uses xe2x80x9cwide-borexe2x80x9d near infrared spectrometry, i.e. wherein the cell path of the solution subjected to the near infrared radiation is at least 3 mm, preferably 3-20 mm, and more preferably 5-12 mm. This clearly distinguishes the invention over prior art methods (27, 28) which teach the use of xe2x80x9cnarrow-borexe2x80x9d path lengths of  less than 2 mm, when measuring the first overtone of the near infrared (approximately 4,000-7,000 cmxe2x88x921), or  less than 1xc3x9710xe2x88x923 cm when measuring the mid-infrared region (approximately 4,000-400 cmxe2x88x921).
The present invention is thus of significant value in providing for the rapid determination of the alkalinity OHxe2x88x92, CO3xe2x95x90 and HSxe2x88x92 levels in pulp liquors, which contains inter alia, all three species in varying amounts, and also for ClO3xe2x88x92, SO4xe2x95x90, S2O3xe2x95x90, polysulfide, and peroxide anions.
Surprisingly, the invention provides that although signal strengths of the water absorption bonds diminish with increasing wavenumber from the infrared to the visible spectral range, increasing the sample path length enables sufficient signal absorption to occur in multi anionic species-containing solutions, within the background noise to enable enhanced accurate spectral data on each of the anionic species to be obtained. Such rapid and accurate anionic species concentration of the order of xc2x11 g/L in pulp liquors allows for good and beneficial control of pulp liquor concentrations.
Cellulosic pulp cooking liquor which has been extracted from the cooking process at some point after coming into contact with the wood chips is collectively referred to as black liquor. The actual composition of any black liquor can vary substantially with a strong dependence on the time and location of extraction, the original composition of the wood and/or liquor upon entering the digester, and the cooking conditions. The dissolved substances in black liquor fall into two primary categories: total inorganic content and total organic content. The inorganic content, which constitutes 25 to 40% of the dissolved substances, consists primarily of anionic species such as hydroxide, hydrosulfide, carbonate, chloride, sulfate, sulfite and thiosulfate, where sodium is the primary counter ion. The organic content, which constitutes the remaining 60 to 75% of the dissolved substances, can be further divided into three main categories: ligninxe2x80x94aromatic organic compounds (30-45%), carbohydratesxe2x80x94hemicelluloses and cellulose degradation products (28-36%), and extractivesxe2x80x94fatty and resinous acids (3-5%). These organic species provide unique contributions to the overall electromagnetic spectral signature of a black liquor sample. Therefore, it is possible to relate the near infrared spectrum of a black liquor sample to the total or constituent organic content of that liquor for calibration purposes. In this way, it is possible to simultaneously measure, for example, the lignin and the sodium hydroxide (or EA) content of a black liquor extracted from a digester. In a more general sense, the total organic content and the total inorganic content, as well as the sum of these two constituents (i.e., the total dissolved solids) would also be quantifiable in a similar manner. Surprisingly, the transmission of near infrared radiation through black liquor is still great enough to quantify these components even when a pathlength of 10 mm is used.
Thus, the present invention provides a rapid method for determining effective alkali, residual effective alkali, sodium sulfide, sodium carbonate, and dead-load components, such as sodium chloride, sodium sulfite, sodium sulfate, sodium thiosulfate and dissolved organic species in process liquors and controlling appropriate parameters in the cellulosic pulp manufacturing process based on the determined values. The proposed method largely eliminates the need for frequent equipment maintenance, sample pretreatment and the use of chemical reagents. High sample throughput can also be obtained by allowing many process streams to be multiplexed to a single analyser through an optional fiber-optic network.
Samples of process liquors are analysed by near-infrared Fourier transform infrared (FT-IR) spectrometry. Spectra are collected using a flow-through wide-bore transmittance accessory. The absorbance of the liquor is measured over a predetermined wavelength region. The absorbance is then correlated through a multivariate regression method known in the art as partial least-squares (PLS) with the concentration of the absorbing compound. This correlation is made by comparing results previously obtained with standard samples. The chemical composition of the liquor is then calculated. The process samples are also analysed with either standard CPPA, SCAN or TAPPI analytical methods, to establish a correlation with the data obtained by near-infrared spectrometry.
The on-line method for EA and REA may primarily be used for controlling the operation of either batch or continuous digesters. The blow-line kappa number can then be predicted by using its well-known relationship with the REA. The method can also be used for controlling carbonate and hydroxide levels in green and white liquors. The causticizing efficiency could also be calculated. In summary, this new sensing and control method could replace automatic titrators and conductivity sensors. It would also give previously unavailable information on the carbonate levels in process liquors, while improving the control of scaling in multi-effect evaporators.
In a preferred aspect, the present invention provides a method for measuring effective alkali in a kraft pulp manufacturing process and controlling the appropriate process parameters said method comprising the steps of:
subjecting samples of process liquors to near infrared radiation at a wavelength region of wavenumbers from about 7,000 to 14,000 cmxe2x88x921 to produce measurements of said liquor;
recording the spectrum of different mixture solutions of synthetic and process liquors having known EA;
correlating by multivariate calibration the relationships between the spectra of the process liquor samples and the different mixture solutions of known EA so as to simultaneously determine EA in the process liquor samples; and
adjusting the cooking conditions selected from time and temperature of the kraft pulp manufacturing process by controlling at least one process parameter to bring said cooking conditions as determined by said near infrared measurements on the process liquor to desired values.
In a further aspect the invention also provides an apparatus for determining the concentration of hydrogen ion and an anionic species selected from the group consisting of OHxe2x88x92, CO3xe2x95x90, ClO3xe2x88x92, SO4xe2x95x90, S2O3xe2x95x90, polysulfide, peroxide and HSxe2x88x92 in an aqueous solution, said apparatus comprising sample means for providing said sample with a solution path length of not less than 3 mm; Fourier transform near infrared means for subjecting said solution over said path length to near infrared radiation at a wavelength region of wavenumbers selected from about 7,000 to 14,000 cmxe2x88x921; and spectral recordal means for recording spectral data of said radiation after subjecting said solution to said radiation.